The present invention describes a cutting tool for metal machining, having a substrate of cemented carbide, cermet, ceramics or high speed steel and, on the surface of said substrate, a hard and wear resistant refractory coating deposited by Physical Vapor Deposition (PVD). The coating is adherently bonded to the substrate and is composed of a laminar, multilayered structure of metal nitrides or carbides in combination with alumina (Al.sub.2 O.sub.3), and with the metal elements of the to nitride or carbide selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W, Al or mixtures thereof. The individual metal nitride (or carbide) and alumina layers have layer thicknesses in the nanometer (nm) range and the stacking of the layers is aperiodic with respect to individual layer thickness.
The process of depositing a thin refractory coating (1-20 .mu.m) of materials like alumina, titanium carbide and/or titanium nitride onto a cutting tool body, e.g., cemented carbides or similar hard materials such as cermets, ceramics and high speed steels, is a well-established technology and the tool life of the coated cutting tool, when used in metal machining, is considerably prolonged. The prolonged service life of the tool may under certain conditions extend up to several hundred percent greater than that of an uncoated tool. Said refractory coatings generally comprise either a single layer or a combination of layers. Modern commercial cutting tools are characterized by a plurality of layer combinations with double or multilayer structures. The total coating thickness varies between 1 and 20 micrometers (.mu.m) and the thickness of the individual sublayers varies between a few microns and a few tenths of a micron.
The established technologies for depositing such coatings are CVD and PVD (see, e.g., U.S. Pat. Nos. 4,619,866 and 4,346,123). PVD coated commercial cutting tools of cemented carbides or high speed steels usually have a single coating of TiN, TiCN or TiAlN, but combinations thereof also exist.
There exist several PVD techniques capable of producing refractory thin films on cutting tools. The most established methods are ion plating, magnetron sputtering, arc discharge evaporation and IBAD (Ion Beam Assisted Deposition). Each method has its own merits and the intrinsic properties of the produced coating such as microstructure/grain size, hardness, state of stress, cohesion and adhesion to the underlying substrate may vary depending on the particular PVD method chosen. An improvement in the wear resistance or the edge integrity of a PVD coated cutting tool being used in a specific machining operation can thus be accomplished by optimizing one or several of the above mentioned properties.
Furthermore, new developments of the existing PVD techniques by, i.e., introducing unbalanced magnetrons in reactive sputtering (S. Kadlec, J. Musil and W.-D. Munz in J. Vac. Sci. Techn. A8(3), (1990), 1318) or applying a steered and/or filtered arc in cathodic arc deposition (H. Curtins in Surface and Coatings Technology, 76/77, (1995), 632 and K. Akari et al. in Surface and Coatings Technology, 43/44, (1990), 312) have resulted in a better control of the coating processes and a further improvement of the intrinsic properties of the coating material.
With the invention of the PVD bipolar pulsed DMS technique (Dual Magnetron Sputtering) which is disclosed in DD 252 205 and U.S. Pat. No. 5,698,314, a wide range of opportunities opened up for the deposition of insulating layers such as Al.sub.2 O.sub.3. Furthermore, this method has made it possible to deposit crystalline Al.sub.2 O.sub.3 layers at substrate temperatures in the range 500.degree. to 800.degree. C. Al.sub.2 O.sub.3 exists in several different phases such as .alpha. (alpha), .kappa. (kappa) and .chi. (chi) called the ".alpha.-series" with hcp (hexagonal close packing) stacking of the oxygen atoms, and in .gamma. (gamma), .theta. (theta), .eta. (eta) and .delta. (delta) called the ".gamma.-series" with fcc (face centered cubic) stacking of the oxygen atoms. The most often occurring Al.sub.2 O.sub.3 phases in CVD coatings deposited on cemented carbides at conventional CVD temperatures, 1000.degree.-1050.degree. C., are the stable alpha and the metastable kappa phases, however, occasionally the metastable theta phase has also been observed. According to U.S. Pat. No. 5,698,314, the DMS sputtering technique is capable of depositing and producing high-quality, well-adherent, crystalline .alpha.-Al.sub.2 O.sub.3 thin films at substrate temperatures less than 800.degree. C. The ".alpha.-Al.sub.2 O.sub.3 " layers may partially also contain the gamma (.gamma.) phase from the ".gamma.-series" of the Al.sub.2 O.sub.3 polymorphs. When compared to prior art plasma assisted deposition techniques such as PACVD as described in U.S. Pat. No. 5,587,233, the novel, pulsed DMS sputtering deposition method has the decisive, important advantage that no impurities such as halogen atoms, e.g., chlorine, are incorporated in the Al.sub.2 O.sub.3 coating.
Conventional cutting tool material like cemented carbides comprises at least one hard metallic compound and a binder, usually cobalt (Co), where the grain size of the hard compound, e.g., tungsten carbide (WC), ranges in the 1-5 .mu.m region. Recent developments have predicted improved tool properties in wear resistance, impact strength, hot hardness by applying tool materials based on ultrafine microstructures by using nanostructured WC-Co powders as raw materials (L. E. McCandlish, B. H. Kear and B. K. Kim, in Nanostructured Materials, Vol. 1, pp. 119-124, 1992). Similar predictions have been made for ceramic tool materials by for instance applying silicon nitride/carbide-based (Si.sub.3 N/SiC) nanocomposite ceramics and, for Al.sub.2 O.sub.3 -based ceramics, equivalent nanocomposites based on alumina.
With nanocomposite nitride/carbide and alumina hard coating materials, it is understood that for a multilayered coating, the thickness of each individual nitride (or carbide) and alumina layer is in the nanometer range between 3 and 100 nm, preferably between 3 and 20 nm. If a certain periodicity or repeat period of the metal nitride/carbide and alumina layer sequence is involved, these nanoscale, multilayer coatings have been given the generic name of "superlattice" films. A repeat period is the thickness of two adjacent metal nitride/carbide and alumina layers. Several of the binary nitride superlattice coatings with the metal element selected from Ti, Nb, V and Ta, grown on both single- and polycrystalline substrates have shown an enhanced hardness for a particular repeat period usually in the range 3-10 nm.